1. Field of the Invention
This invention relates to releasing organopolysiloxane compositions and more particularly, to radiation-curable releasing silicone composition comprising methacrylic or acrylic functional group-containing organopolysiloxanes.
2. Description of the Prior Art
Electron radiation-curable silicone compositions are known, for example, in Japanese Patent Publication No. 52-31910. The composition comprises organopolysiloxanes having a (meth)acryloxy group in the molecule. Japanese Patent Publication No. 53-36515 sets out the composition comprising such organopolysiloxanes as mentioned above, to which sensitizers or photoinitiators are further added. These types of compositions are stated as having more improved radiation curability as the content of the (meth)acryloxy group in the organopolysiloxane becomes larger.
However, when cured, the compositions exhibit a high density of crosslinkage. This undesirably leads to a high hardness of the cured film with the lowering of a release characteristic relative to self-adhesive materials. As a consequence, an abnormally great release force will be undesirably required. In addition, the follow-up action of the cured film on substrates becomes worsened, with the attendant drawback that the film is liable to crack, adversely influencing the release characteristics.
In order to make a cured film whose hardness is low, there has been proposed use of an organopolysiloxane which has one (meth)acrylic group at each terminal end without any (meth)acrylic group being joined to side chains of the siloxane units. However, the cured film obtained from such an organopolysiloxane does not have a low hardness and good releasing properties against self-adhesive substances without a sacrifice of curability.
Preparation of the above type of organopolysiloxane having (meth)acrylic functionality has also been proposed by use of a process of dehydrochlorination from .tbd.Si--Cl groups and a polyacrylate such as pentaerythritol triacrylate. This process is set forth, for example, in Japanese Patent Publication No. 58-53656. The (meth)acrylic functionality-bearing organopolysiloxane obtained by this process has the siloxane chains and the (meth)acrylic group joined through chemically unstable .tbd.Si--O--C.tbd. linkage. This is disadvantageous in that the organopolysiloxane and the cured product obtained therefrom suffer a great change in properties in relation to the time.
Moreover, Japanese Laid-open Patent Application No. 63-135426 has proposed a process of preparing an organopolysiloxane having both (meth)acrylic functionality and epoxy functionality in which an organopolysiloxane having epoxy functionality is first reacted with (meth)acrylic acid and then with (meth)acrylic acid chloride. In order to obtain this type of epoxy functional organopolysiloxane, an organohydrogenpolysiloxane is first prepared, followed by hydrosilylation reaction with an epoxy compound having an alkenyl group in the presence of platinum. Thus, the process undesirably requires multi-stage reactions.